The present invention relates to percussive drilling equipment in which a hammer piston is impacted against a drill bit under the urging of a flow of flushing fluid such as air or water, and wherein flushing fluid is delivered to the front of the drill bit to cool and clean the cutting surface and flush cuttings from the bore.
In percussion drills, a drill bit mounted at the front end of a drill string is rotated and longitudinally impacted in order to cut through hard earth formations such as rock. The longitudinal impacts are transmitted from a reciprocable hammer piston driven by compressed fluid such as water or air which is supplied through a supply passage and applied alternately to front and rear ends of the piston to produce reciprocation of the piston. Surplus fluid, i.e., fluid over and above that needed to reciprocate the piston, flows forwardly to the front end of the drill bit in order to cool and clean the cutting elements. This fluid is termed flushing fluid because it also entrains the cuttings and flushes them from the bore hole through a passage spaced radially from the fluid supply passage.
During periods when no flushing fluid is being supplied to the drill bit, it is desirable to prevent a backflow of subterranean liquid through the fluid supply passage which could result in cuttings and other debris fouling, damaging the tool. For example, smaller cuttings could work their way to a position between the piston and outer casing, causing accelerated abrasive wear. The risk of liquid backflow typically exists mainly during upwards drilling, i.e., when the drill bit is advancing in an upwards direction, since gravity tends to promote such a backflow. Upwards drilling is performed, among other reasons, to interconnect vertically spaced mine shafts.
The backflow problem can also occur during downwards drilling, when the upward pressure of fluid disposed in the bore hole, such as subterranean fluid (e.g., a water and sand mixture) or accumulated flushing fluid (e.g. water, drilling mud etc.), is great enough to produce a backflow of the fluid past the drill bit.
Accordingly, it has been the practice to provide the fluid supply passage with a spring-biased check valve to prevent a backflow of subterranean liquid. In FIG. 4, for example, a conventional drill bit 2 includes a fluid supply passage 4 for conducting flushing fluid. The fluid supply passage includes a main portion 5, and a plurality of branch lines 6 extending to the front end of the drill bit from a front end of the main portion 5. A check valve disposed in the main portion 5 includes a ball 8 and a spring 10 yieldably biasing the ball towards a seat 12 to prevent a backflow of fluid from the drilling face. During a drilling operation, flushing fluid forces the ball 8 off the seat by compressing the spring 10 as shown in FIG. 4. A drawback to that check valve is that the ball remains directly in the path of fluid flow, causing the size of that path to be diminished. The cross-sectional size of the main portion 5 has to be made large enough so that the space between the ball 8 and a wall 14 of the passage can conduct the requisite amount of flushing fluid. However, the more material of the drill bit which is removed, for that purpose, the more the drill bit is weakened.
Also, the pressure of the fluid will tend to fluctuate, whereby the amount by which the spring is compressed will vary. That is, the spring will flex in response to the pressure fluctuations, and such flexing produces metal fatigue and possible spring breakage.
In Canadian Patent No. 853,062 there is disclosed a check valve in the shape of a flat disk having a central through-hole formed therein. The disc is mounted for free vertical movement on a guide pin which extends axially within the fluid passage, the guide pin passing through the through-hole. It is intended that the disk be positioned against a front seat during downwards drilling, to enable flushing fluid to travel past the disk en route to the front drilling face. In the event that liquid from the drilling face backs up into the fluid passage during a cessation of the flow of flushing fluid, it is intended that the disk float upwardly upon that liquid and into sealing contact with a rear seat to block further backflowing of the liquid. However, a potential problem is that debris such as cuttings may become lodged in the through-hole of the disk and thus opposes movement of the disk. In that event, it may be possible for the liquid and entrained particles to rise high enough to reach and abrade the percussion elements. Moreover, the presence of the guide pin in the fluid passage presents an obstacle to the downward flow of flushing fluid. Also, it becomes necessary to create a fluid seal between the through-hole and guide pin to prevent well fluid from flowing upwardly past the disk when the disk is in its upward sealing position. In addition, it is necessary to ensure that the guide pin remains in an axial orientation despite the forces imposed thereagainst, e.g., by the flow of flushing fluid; otherwise, the disk cannot create a proper seal. Hence, the overall structure of the check valve becomes more complicated, expensive, and subject to wear.
Therefore, it would be desirable to provide a check valve for a percussion drill which does not appreciably diminish the size of the fluid passage and does not involve a risk of spring breakage.